Dual polarized radar systems are well known for detecting targets in marine as well as other detection environments. Under normal operating conditions, a radar transmits a horizontally polarized pulse and receives both horizontal U.sub.hh (like-polar) and vertical Y.sub.hv (cross-polar) signals. However, such systems typically suffer from a cross-polar signal leakage problem due to performance limitations in the antenna and rotary joint of such prior art systems, as well as depolarization of the returned signal by the environment. The leakage transfer function can be modelled as a zero order system with varying gain.
One prior art technique for cancelling the cross-polar signal leakage comprises the step of simply subtracting the like-polar channel from the cross-polar channel arithmetically, thereby providing a measure of interference cancelling. However, as discussed above the signal leakage components typically vary depending on environmental conditions, etc. Thus, the simple arithmetic subtraction technique does not result in complete cancellation of the interference component.
U.S. Pat. No. 4,106,014 (Dunn) discloses a target signature analysis system for differentiating between targets based upon different effects of the return polarized signal. The pulse radar set transmits signals at +45.degree. linear polarization and receives both +45.degree. and -45.degree. linear polarized signals. A detection is registered when the orthogonal polarization exceeds the like polarization return signal. This occurs primarily for horizontal and vertical dipole-like edges found in military vehicles.
The main thrust of the radar set as disclosed by Dunn is to find vertical and horizontal dipoles in the environment. The method used is not adaptive to the environment and depends on the orthogonal polarization exceeding that of the like polarization return signal.
U.S. Pat. No. 4,490,719 (Botwin et al) d missile guidance system which uses a process for comparing a horizontally polarized radar map to a vertically polarized radar map, noting which points on the map have equal returns for both polarizations, and then conveying these points on to a map matcher. The resultant information is used by the missile guidance system.
The Botwin et al system is based on the assumption that man-made objects return horizontal and vertical polarized signals roughly equal. Since man-made objects are often good navigational references, this information can be used within a missile guidance system. However, there is no disclosure of enhancing the difference in polarization and suppressing commonality between horizontal and vertical channels.
U.S. Pat. No. 4,035,797 (Nagy) discloses a polarized radar system for determining the complexity of targets (e.g. automobiles) in order to identify and discriminate between them. The system uses two quadrature frequencies and orthogonal polarizations to create a complexity measure of a received target signal. The output of the system is a sum of the absolute values of the various polarization ratios, the derivative of which is normalized by the range rate of change of the target.
Although the system of Nagy is sensitive to polarization, it uses the ratios between different polarizations to create a discriminant based on change of polarization information in range.
U.S. Pat. No. 4,028,697 (Albanese et al) describes a dynamic signal processing system using an adaptive filter to achieve enhanced target discrimination in clutter. Processing of the signal involves a time varying filter which can adapt to changes in clutter spectrum. Although the Albanese et al patent discloses means for processing the return signals to achieve a maximum signal to interference ratio, it does not utilize any information in a cross polarization channel. The adaptive time varying filter only makes use of correlations in the like channels, and therefore is not relevant to the problem of cross-polar signal leakage.